Copper plating



Patented June 23, 1942 COPPER PLATING Christian J. Wernlund, North Tonawanda, and Harry Lloyd Benner and Robert Richard Bait, Niagara Falls, N. Y., assignors to E. I. do Point -de Nemours & Company, Wilmington, Del., a

corporation of Delaware No Drawing. Application May 4, 1938, Serial No. 206,002

15 Claims- (Cl. 204-52) This invention relates to the electroplating of copper and more particularly to the electrodeposition copper from cyanide baths.

Electroplated copper coatings are used extensively as undercoats for other metals such as nickel and chromium for the formation of smooth, bright finishes on base metals such as iron, steel, zinc alloys and the like. In order to obtain such bright finishes, it is ordinarily necessary to bufl or polish the copper undercoating before applying the top coating. The bufling operations are often difficult and costly and require skilled operators. It is desirable to be able to directly electroplate smooth. lustrous, copper coatings in order to eliminate the bufilng operation. Heretofore various methods have been proposed for producing bright copper coatings of this sort but ,none of these have enjoyed substantial commercial use. Prior to the present invention, it has been diflicult to electroplate bright copper at reasonably high current densities and at high current efllciencies in an economical manner.

An object of the present invention is to directly electroplate a bright copper electrodeposit which has substantially the characteristics of the polished'copper surface. A further object is to electroplate such bright copper at relatively high current densities and with high current efficiency. Another object is to provide an improved process for electroplating copper. pparent from the following description of our invention.

The above objects may be attained in accordance with our invention by electroplating copper from a solution of the double cyanide of copper and an alkali metal, which solution also contains an alkali metal sulfocyanide, an alkali metal hydroxide, and preferably also a small amount of a soluble carbohydrate. Preferably, the bath is operated at a bath temperature of about 60 C. up to the boiling point of the solution.

We are aware that copper, silver and other metals heretofore have been electroplated from solutions containing sulfocyanides. However, none of these prior methods with which we are acquainted include the essential features of our herein described invention, nor are they capable of producing smooth, bright coatings of copper equivalent to the coatings produced by our method.

As an illustration of our invention we may employ a hath. made by dissolving together around 16 ounces per gallon of copper cyanide and an equal amount of sodium cyanide, together with about 4 ounces per gallon of sodium hy- Other objects will be r droxide. A brightening agent is prepared by heating a starch or other carbohydrate dissolved in a sodium sulfocyanide solution and this brightening agent is then added to the solution. The proportions of carbohydrate and sulfocyanide are selected so as to add to the electroplating solution a small fraction of one ounce per gallon (e. g. 0.1 ounce per gallon) of the carbohydrate and 1 to 8 ounces per gallon of the sodium sulfocyanide (NaCNS). The resulting electroplating solution then is operated with a bath temperature of to C. and a cathode current density of 10 to 30 amperes per square foot, using a copper anode. The resulting copper electrodeposit, which may be plated directly onto iron, steel, zinc alloy or other desired base metal, is smooth and bright, resembling a highly polished copper surface.

The invention is further illustrated by the following speciflc examples.

Example I A copper plating bath was made up according to the formula:

Ounces Der gallon Sodium cyanide 13.5 Copper cyanide (CuCN) 12.0 Sodium hydroxide 1.0 Sodium sulfocyanide 2.0

ties were:

Current density Current efiiciency Run Anode Cathode Anode Cathode Amps. per Amps; per

It. sq. ft. Per cent Per can A 7. 5 15 100. 2 99. 6 B 15 30 100. 0 99. 6 C 25 50 100. 3 99. B

above. In all three runs the cathode deposit was smooth and bright.

The batch temperature was maintained at (a) 78 C. and (b) at 60 C. The anodes were electrolytic copper; the cathodes, cold rolled sheet steel. The ratio of anode to cathode surface area was 3 to 1. The bath was operated in different runs at the above mentioned temperatures at various current densities. In some runs the cathodes were kept in motion. as in Example I,

at a linear speed of 50 inches per minute. Ex-

oellent bright plate was obtained under the following conditions:

Bath temperature Cathode current density to 30 amps. per sq. it.stationary cathode. to 40 amps. per sq. it.--moving cathode. to 20 amps. per sq. lt.stationary cathode. to 30 amps. per sq. it.--moving cathode.

Example III In this example, the bath composition was the same as in Example II, except that gum arabic was used in place of the starch. In two series of experiments, the gum arabic concentrations were 4.8 grams per gal. and 12 grams per gal. respectively. The gum arable was added in the form of a solution prepared by dissolving the gum arabic in a boiling solution of sodium hydroxide, 4% and sodium sulfocyanide, 6% In each series, the bath was operated at various current densities, at a bath temperature of 75 C., both with stationary cathodes and with cathodes moving at 50 inches per minute as in the preceding examples. Good bright plate was obtained under the following conditions:

Gum arable concen- Cathode current density 12 to 30 amps. per sq. ft.-stationary cathode. 15 to 40 amps. per sq. ft.moving cathode. 12 to 32 amps. per sq. ft.stationary cathode. 15 to 50 amps. per sq. (la-moving cathode.

Example IV The electroplating solution was made by solving together:

Sodium cyanide ounces per gallon 16 Copper cyanide do 16.5 Sodium hydrmrirln dn 4 Potassium sulfocyanide do 2 Cane sugar grams per gallon" 4 The bath was operated at 80 C. and produced good bright copper plate on stationary sheet steel cathodes at cathode current densities as high as 32 amps. per sq. ft.

Example V Epample VI A marked improvement in the brightness and bright plating current density ranges was obtained by using specially prepared starch solutions prepared as described below.

Solution No.

Damper 0s. per 02. per Oz. per 02. per a aal. gal. pal. anl. Corn starch 6. 7 10. 7 5. 3 6. 7 6. 7 NBCNS 20. 0 10. 7 NaOH 1.2 m 5.3 NaCN 6. 7 a 7 CnCN (i. 7

Sodium cyanide ounces pergallon-.. 16 Copper cyanide do .,,.;16 Sodium hydroxide do- Sodium sulfocyanide do 2 Starch do 0.12

These baths, operated at about 70 C. gave excellent bright plate on stationary steel cathodes at a cathode current density of 27 amps. per sq. ft.

The above examples are illustrative only. Various modifications may be made in the invention without departing from the spirit and scope of our invention. The essential features of the invention are that: (1) the copper be present in the electrolyte as the double cyanide of copper and alkali metal, (2) the total amount of alkali metal cyanide, both free and combined, does not exceed that equivalent to a sodium cyanide to copper cyanide ratio of 1.25 to 1, (3) the alkali metal hydroxide concentration be not less than 1 ounce per gallon, and (4) the electrolyte contain not less than 1 ounce per gallon of an alkali metal sulfocyanide (e. g., NaCNS) Further, while we prefer to use sodium cyanide, hydroxide and sulfocyanide because of their cheapness, the corresponding compounds of the other alkali metals, e. g. potassium or lithium, may be used as well.

Under certain conditions, we may obtain the" bright copper deposit without adding the carbohydrate, for example, by using a sufllciently low current density. For practical purposes, this generally requires a high concentration of copper (e. g. 12 to 20 ounces per gallon of CuCN) and that the bath be operated hot. Even under such favorable operating conditions, the process is much less satisfactory than when the carbohydrate addition is utilized. Without the carbohydrate addition, operating conditions must be very carefully controlled to consistently produce the bright plate and hence we prefer to make the carbohydrate addition. However, without the carbohydrate addition, the bath is excellent- 1y adapted for plating non-bright coatings, es-

for adding to the electrolyte in accordance with the present invention. Any carbohydrate which is soluble in water (e. g., a sugar) or which forms colloidal solutions in water (e. g., starch or gum arable) is suitable for our purpose. For the sake of brevlty.'throughout this specification and the appended claims, the term soluble carbohydrate" is used to denote those carbohydrates which form in water either true solutions or colloidal solutions. We have found, for example, that the following carbohydrates give excellent results: corn starch, potato starch, cane sugar, honey, mannite, molasses, and gum arabic. As

will be seen from the above examples, it is not essential that the carbohydrate be in a pure state, for such complex carbohydrate materials as honey and molasses may give results equally as good as those obtained with pure carbohydrates. We prefer to use starch, as we have found that generally better results are obtained with starch than with the more soluble carbohydrates such as cane sugar and the like.

We also prefer to heat the carbohydrate with the alkali metal sulfocyanide or other alkaline ingredient of the plating bath in water solution before adding these ingredients to the electroplating bath. It is not necessary to heat the entire amount of sulfocyanide to be used in the bath with the starch to be added to the electroplating solution. In general, however, the amount of sulfocyanide heated with the starch should be at least of equal Weight with that of the starch taken and preferably from 2 to 3 times that amount. In heating thestarch with the sulfocyanide solution, it is not essential that the starch be dissolved prior to the heating. The starch in the sulfocyanide solution should be heated for at least about minutes at a temperature of 60 to 100 C.; preferably, we heat the mixture at 95 to 100 C. for to 30 minutes. At temperatures below 95 C., the time of heating should be longer, usually 30 to 60 minutes. Substantially equivalent results can be obtained if the starch is heated with a water solution of oneof the other alkaline ingredients of the plating bath, e. g., with a solution of sodium hydroxide or sodium cyanide in place of the sulfocyanide solution. Likewise, the starch may be heated with solutions containing one or more of the three bath ingredients alkali metal sulfocyanide, alkali metal hydroxide and alkali metal cyanide. The resulting carbohydrate-containing solution or mixture may then be added to the electroplating bath so as to add from about 0.06 to 1 ounce per gallon of the carbohydrate to the bath. When starch is used, we prefer to add not more than about 0.33 ounce per gallon of starch to the plating bath. The optimum concentration will vary somewhat for the different carbohydrates and for a given carbohydrate this readily may be determined by experiment.

If desired, a large quantity of the carbohydrate addition agent may be prepared by the above described heating with water and one or more of the alkaline bath ingredients: alkali metal cyanide, alkali metal hydroxide or alkali metal sulfocyanide. The resulting addition agent then may be used as required for diiferent'plating baths.

Alternately, the carbohydrate may be added directly to the electroplating bath, provided that the bath is heated at an elevated tem erature,

e. g. 60 to 90' C. However, when the carbohydrate is so directly added, the benefit of its presence is not realized until after it has been heated in the bath for a period of time which .may vary from 15 minutes to one or two hours.

In either case, the carbohydrate, when heated with one or more of the aforesaid alkaline bath ingredients, appears to react therewith or be modified thereby to form a reaction product which acts as brightening agent in our process. While such reaction or modification of the carbohydrate appears to occur to some extent in the cold, -it is hastened and made more complete by heating the solution to a temperature of 60 C. or higher.

In order to produce smooth bright deposits of copper in accordance with our invention, we prefer to Operate the bath at an elevated temperature, that is, from about 60 C. up to the boiling point of the solution. Ordinarily we prefer to maintain the bath temperature of 70 to C. If the electroplating bath is operated in the cold, excellent, compact, adherent deposits of pure copper are readily obtained over a wide range of current density but ordinarily these will not be smooth and bright. On the other hand, if the bath is operated at 60 C. or h gher, smooth, bright electrodeposits are readily obtained, providing the current density is maintained within reasonable limits as explained be- In order to obtain the bright copper at a reasonably high cathode current density it is essential that the composition of the bath be maintained within certain limits. The copper should be present substantially in the form of a copper alkali metal double cyanide and the concentration of the copper in the solution should be equal to at least about 8 ounces per gallon of copper cyanide and not more than about 2 2 ounces per gallon of copper cyanide. Outside of this copper concentration range, it is still possible to, obtain the bright copper deposits, but the current density range must be narrowed accordingly.

.For best results, there should be little or no free cyanide in the bath, e. g., not over one ounce per gallon. By "free cyanide, we mean the alkali metal cyanide concentration in excess of that requiredto combine with the copper cyanide to form the double cyanide represented by MzCu(CN)a where M represents alkali metal, e. g.,

NazCu(CN)a For example the free cyanide content should not be over 3 to 4 ounces per gallon in asolution containing 16 ounces per gallon of copper cyanide. In our process, the ratio of sodium cyanide to copper cyanide, regardless of copper concentration, must not exceed 1.25 to 1; preferably a, ratio of 1:1 is used. I For other alkali metal cyanides,

wcights per gallon of these two ingredients, which represents somewhat less sodium cyanide than is required to convert all the copper cyanide to the double cyanide Na2Cu(CN)3. We have found that with these proportions, all of the copper cyanide goes into solution, possibly because of the conversion of part of the copper cyanide into the lower double cyanide NaCu(CN) 2. In general, we prefer to use the minimum amount of alkali metal cyanide required to completely dissolve the copper cyanide.

The bath must contain an appreciable amount of alkali metal hydroxide and in our preferred modification we add from 3 to 8 ounces per gallon of alkali metal hydroxide to the bath. Usually, an alkali metal hydroxide concentration of about 4 ounces per gallon gives the best results. In any event, in order to obtain the bright copper deposits at any feasible current density range, it is essential to have in the bath at least 1 ounce per gallon of alkali metal hydroxide. Further, the concentration of this ingredient should not rise above about 8 to 10 ounces per gallon; otherwise, oxide coatings tend to form on the anodes to such extent as to seriously interfere with anode corrosion, even at low anode current densities. We have found that ammonium hydroxide is not suitable as an alkalining agent for our solution. Also alkaline carbonates such as sodium carbonates cannot be used as the sole alkalining agent. The addition of carbonate to the bath which contains alkalimetal hydroxide ordinarily is not seriously harmful. However, the presence of a substantial amount of carbonate in a copper plating bath is usually objectionable and we prefer not to add carbonate in any material amount.

The amount of alkali metal sulfocyanide may be varied between 1 to 8 ounces of sodium sulfocyanide per gallon. We prefer to use about 2 ounces per gallon of this ingredient. The carbohydrate concentration may vary from 0.06 to 1 ounce per gallon, depending on the carbohydrate used; preferably, it is maintained at between 0.12 to 0.33 ounce per gallon.

The limiting current densities at which the smooth bright copper plate may be obtained will vary depending upon the bath concentration. Also, the current density range will be different for a moving cathode than for a stationary cathode. Using a hot bath, that is at 60 C. or higher, and a stationary cathode, the cathode current density'may be as high as 30 amperes per square foot for a concentrated bath containing around 16 to 20 ounces per gallon of copper cyanide. However, if the bath contains about 8 to 10 ounces per gallon of copper cyanide, the cathode current density should not exceed about 10 to amperes per square foot in order to deposit bright copper. If the cathode is moved in such a manner as to agitate the electrolyte layers in contact with the cathode, much higher cathode current densities can be used in a given bath. Such agitation may be obtained by moving the cathode surface with respect to the electrolyte in any desired mechanical manner. For examplethe cathode may'be moved either horizontally or vertically in a reciprocal motion or, if cylindrical, the cathode may be rotated. Alternately, the cathode may be stationary and by means of suitable conduit and pumping device a stream of the electrolyte may be flowed over the cathode surface at the desired rate. In general, the greater the agitation thus obtained at the cathode surface, the higher the cathode current density may be permitted to rise without sacrificing the brightness of the cathodic deposits. For example, by rapidly rotating a cylindrical cathode, we have obtained smooth bright copper deposits at a cathode current density as high as about 100 amperes per square foot in a bath containing around 16 ounces per gallon of copper cyanide. In other tests where the cathodes were moved in a reciprocal motion at a relatively slow rate in a bath of the same concentration, the maximum allowable cathode current density was about 50 amperes per square foot. In general we prefer to employ a moving cathode whenever feasible, in order to take advantage of the higher permissible cathode current densities.

The anode current density likewise is dependent upon the electrolyte concentration and also upon the bath temperature. For example in a hot bath containing 8 to 10 ounces per gallon of copper cyanide, the anode current density should not exceed about 15 amperes per square foot, while at higher concentrations, e. g. 12 to 20 ounces per gallon of copper cyanide, the anode current density in a hot bath may be raised to as high as amperes per square foot. If the bath is operating cold these figures must be much lower, e. g. 2 to Gamperes per square foot. In either the hot or cold bath, if the anode current density is allowed to exceed these limits, an insoluble film tends to coat over the anode thus preventing its solution.

The limiting current densities at both electrodes also will vary to some extent depending upon the particular carbohydrate used as addition agent and its concentration and also on the alkalinity of the bath and other factors. Thus, if the amount of alkali metal hydroxide in the bath is relatively low, the current density bright plating range will be narrowed somewhat.

The above described limiting cathode current densities are given to show how the current density must be controlled in order to produce the bright copper deposits. The invention is not restricted to the numerical limits of current density given, which merely show the limitations of the process, insofar as the production of the bright deposits is concerned, under the particular conditions we have explored. Further, above these current density limits, the process is cperative to produce compact, adherent copper deposits of excellent character, though not bright. Hence, outside the current density range required for plating bright copper, our process and electrolyte is useful for plating copper for a variety of purposes.

An important advantage of our process is that i over extended periods of operation, even at the elevated temperatures, there is little or no cyanide decomposition or attendant carbonate formation. Ordinarily, in platin copper from cyanide baths as heretofore practiced, the decomposition of cyanide to form carbonate gradually decreases the bath efficiency and finally, the carbonate must be removed or a new bath made up. A further advantage is that our process is operative at reasonably high current densities to produce smooth, lustrous deposits of copper, which require no bufling. The bath has good throwing power, so that recessed or irregularly shaped articles may be uniformly coated with the bright copper coating. A further and very important advantage of our process is that within the limiting current densities given above we obtain substantially anode and cathode giinide-copper cyanide ratio current efliciencies, which results in economical, stabilized plating solutions from which copper can be deposited very rapidly and efliciently with little or no change in bath composition. So far as we know, heretofore commercial copper plating processes have not exceeded 90% current eiflciency and generally do not exceed 40 to 80%. K

We claim:

1. The process which comprises electroplating copper from a solution containing the double cyankle or copper and an alkali metal, at least '1 oz. per gallon of an alkali metal hydroxide, 1 to 8 oz. per gallon of an alkali metal sulfocyamde and a small amount of a soluble carbohydrate.

2. The process which comprises electroplating copper from a solution containing the double cyanide of copper and sodium, 1 to oz. per gallon oi. sodium hydroxide, 1 to 8 oz. per gallon oi sodium sulfocyanide and a small amount of starch.

3. The process which comprises electroplating copper from a solution containing the double cyanide of copper and an alkali metal, at least 1 oz. per gallon of an alkali metal hydroxide, 1 to 8 oz. per gallon of an alkali metal suliocyanide and a small amount of a soluble carbohydrate, while maintaining said solution at a temperature not lower than about 60 C.

4. The process which comprises electroplating copper from a solution containing 8 to 22 oz. per

gallon of copper cyanide, an approximately equal weight of alkali metal cyanide, about 3 to 8 oz. per gallon of alkali metal hydroxide, 1 to 8 oz. per gallon of alkali metal suliocyanide and about 0.06

to 1 oz. per gallon of a soluble carbohydrate.

5. The process which comprises electroplating copper from a solution containing 8 to 22 oz. per gallon of copper cyanide, an approximately equal weight of sodium cyanide, about 3 to 8 oz. per gallon of sodium hydroxide, 1 to 8 oz. per galion of sodium suliocyanide and about 0.06 to 0.33 oz. per gallon of starch, while maintaining said solution at a temperature of 60 to 100 C.

8. The process which comprises electroplating copper from a solution containing copper cyanide, alkali metal cyanide, alkali metal hydroxide and 1 to 8 oz.,per gallon of alkali metal sulfocyanide, the concentrations oi the alkali metal and copper cyanides being equivalent to a sodium 7. The process which comprises electroplating copper from a solution containing copper cyanide, sodium cyanide, sodium hydroxide-and 1 to 8 oz. per gallon of sodium suliocyanide, the ratio of the sodium cyanide concentration to the cop- 1111:; 1cyanide concentration being substantially 8. The process which comprises electroplating copper from a solution containing 8' to 22 oz.

not exceeding 1.25

per gallon of copper cyanide, an approximately equal weight of sodium cyanide, about 3 to 8 oz. per gallon of sodium hydroxide, and 1 to 8- oz. per gallon of sodium suliocyanide, while maintaining said solution at a temperature of 60 to C.

9. A copper cyanide electroplating bath comprising in aqueous solution copper cyanide, alkali metal cyanide in an amount sufficient to form the copper double cyanide but not more than about 4 oz. per gallon in excess of the amount required to form said double cyanide, at least 1 oz. per gallon of alkali metal hydroxide, 1 to 8 oz. per gallon of alkali metal suliocyanide and a small amount of a soluble carbohydrate.

10. A copper cyanide electroplating bath comprising in aqueous solution a copper sodium double cyanide equivalent to 8 to 22 oz. per gallon of copper cyanide, about 3 to 8 oz. per gallon of sodium hydroxide, 1 to 8 oz. per gallon of sodium suli'ocyanide and a small amount of a soluble carbohydrate.

11. A copper cyanide electroplating bath comprising in aqueous solution 8 to 22 oz. per gallon of copper cyanide, sodium cyanide in an amount approximately equal in weight to that of the copper cyanide, 3 to 8 oz. per gallon of sodium hydroxide, l to 8 oz. per gallon 01 sodium sulfocyanide and 12. A copper cyanide electroplating bath comprising copper cyanide, alkali metal cyanide, alkali metal hydroxide and 1 to 8 oz. per gallon of alkali metal sulfocyanide, the ratio of the concentrations of the copper and alkali metal cyanides being equivalent to a sodium cyanide to copper cyanide ratio of not more than 1.25 to 1.

13. A copper cyanide electroplating solution comprising 8 to 22 oz. per gallon of copper cyanide, an amount of alkali metal cyanide equivalent to a sodium -cyanide concentration which is not more than 1.25 times the concentration of said copper cyanide, at least 1 oz. per gallon of alkali metal hydroxide and 1 to 8 oz. per gallon of alkali metal suliocyanide.

14. A copper cyanide electroplating solution comprising 8 to 22 o per gallon of copper cyanide, an approximately equal concentration of sodium cyanide, at least -1 oz. per gallon of sodium hydroxide and 1 to 8 oz. per gallon of sodium suli'ocyanide.

15. The process which comprises electroplating copper from a solution containing 8 to 22 oz. per gallon oi. copper cyanid an amount of alkali metal cyanide equivalent to a concentration of sodium cyanide which is not more than 1.25 times the concentration of said copper cyanide, at least 1 oz. per gallon oi an alkali metal hydroxide and 1 to 8 oz. per gallon of an alkali metal suli'ocyanide.

0.06 to 0.33 oz. per gallon of starch. 

